docs: add CNCF Slack join instructions (#3555)

Author: cijothomas

CONTRIBUTING.md

@@ -9,8 +9,10 @@ meeting for this group.
 
 Meeting notes are available as a public [Google
 doc](https://docs.google.com/document/d/12upOzNk8c3SFTjsL6IRohCWMgzLKoknSCOOdMakbWo4/edit).
-If you have trouble accessing the doc, please get in touch on
-[Slack](https://cloud-native.slack.com/archives/C03GDP0H023).
+If you have trouble accessing the doc, please get in touch on the
+[#otel-rust](https://cloud-native.slack.com/archives/C03GDP0H023)
+channel on CNCF Slack. If you are new to the CNCF Slack community,
+you can [create an account](https://slack.cncf.io/).
 
 The meeting is open for all to join. We invite everyone to join our meeting,
 regardless of your experience level. Whether you're a seasoned OpenTelemetry

README.md

@@ -168,8 +168,10 @@ meeting for this group.
 
 Meeting notes are available as a public [Google
 doc](https://docs.google.com/document/d/12upOzNk8c3SFTjsL6IRohCWMgzLKoknSCOOdMakbWo4/edit).
-If you have trouble accessing the doc, please get in touch on
-[Slack](https://cloud-native.slack.com/archives/C03GDP0H023).
+If you have trouble accessing the doc, please get in touch on the
+[#otel-rust](https://cloud-native.slack.com/archives/C03GDP0H023)
+channel on CNCF Slack. If you are new to the CNCF Slack community,
+you can [create an account](https://slack.cncf.io/).
 
 The meeting is open for all to join. We invite everyone to join our meeting,
 regardless of your experience level. Whether you're a seasoned OpenTelemetry